Adam Mosseri:
Second, threads posted by me and a few members of the Threads team will be available on other fediverse platforms like Mastodon starting this week. This test is a small but meaningful step towards making Threads interoperable with other apps using ActivityPub — we’re committed to doing this so that people can find community and engage with the content most relevant to them, no matter what app they use.
What communities are you talking about specifically? I tried but I haven’t yet found a list of those that defederated preemptively. I can’t imagine it’s the majority of instances/users.